1. Field of Invention
This invention relates generally to pressure transducers of the type wherein a variable fluid pressure applied thereto gives rise to a corresponding change in capacitance value. Variations in capacitance are convertible into an electrical signal which may be indicated, recorded or transmitted. More particularly, the invention deals with a capacitive pressure transducer which includes a coupling assembly to hydraulically convey the variable fluid pressure to the pressure sensor in a manner isolating the fluid from the sensor structure.
2. Background of Invention
Capacitive pressure transducers are known which include a pressure sensor constituted by two or more plates separated by fluid acting as a dielectric medium, the capacitance value of the sensor being a function of the separation between the plates. The application of fluid pressure to one of the plates which functions as a diaphragm results in a change in separation which is translated into a change in capacitance value.
To convert a change of capacitance value into a corresponding electrical signal for recording, indicating or transmitting changes in pressure, the capacitive pressure transducer may be connected to a capacitance bridge, or it may be included in the tuned resonance circuit of a frequency or phase-modulation system to produce an output signal whose frequency or phase is varied as a function of the applied pressure.
When the fluid pressure to be measured is that encountered in an industrial chemical process, one must take into account the fact that the fluid may be highly corrosive and that its temperature as well as its pressure may vary. To render the pressure transducer relatively immune to high temperature fluids, it is known to form the body and the diaphragm of the transducer from Invar steel and to include a water cooling system in order to minimize the effect of temperature changes on the diaphragm. This cooling serves to maintain the temperature of the transducer within the range in which the low thermal coefficient of Invar is realized. A water-cooled capacitive pressure transducer of the above-described type is disclosed in Section 3-56 of the "Process Instruments and Controls Handbook"--Considine--McGraw Hill Book Company, Inc., 1957.
A water-cooled pressure transducer having an Invar construction is not only costly to manufacture, but the associated water cooling system further complicates and contributes to the overall expense of the installation. Moreover, the metal used in this transducer may not be compatible with many of the corrosive fluid chemicals whose pressures are not to be measured.
The present invention is of particular value in conjunction with a variable-capacitance sensor of the type disclosed in the Lee, et al. U.S. Pat. No. 3,859,575. This sensor includes two capacitance plates connected together at their center to minimize alignment problems and to reduce sources of thermal shift. The Lee et al. arrangement also substantially obviates capacitance shifts due to mounting stresses and acts to enhance the capacitance change resulting from a given pressure-produced motion.
In the Lee et al. pressure sensor, a deformable plate encloses one end of a cylinder into which is introduced the fluid to be metered. The plate formation is such that its central zone dilates in response to an increase in fluid pressure, the annular zone surrounding the central zone and integral with the periphery of the cylinder serving as the first plate of a capacitor. Positioned above the deformable plate and external to the cylinder is a structure constituted by a disc whose center is attached by a stud to the center of the deformable plate. The disc is spaced from the deformable plate by a washer, the disc being encircled by an insulating ring which in turn is surrounded by a conductive ring.
The conductive ring which functions as the second plate of the capacitor is spaced by an air gap from the corresponding annular zone on the deformable plate which functions as the first plate of the capacitor. In operation, an increase in fluid pressure within the cylinder causes the central zone of the deformable plate to dilate and thereby raise the position of the second plate relative to the first plate to lower the capacitance of the sensor.
Though the Lee et al. sensor has many advantages over conventional capacitive pressure transducers, it is not suitable for metering corrosive fluids of the type often encountered in industrial chemical processes, for the metal from which the deformable plate and cylinder of the sensor is made is in some instances reactive with these fluids. And while it would be possible to select a metal or alloy for this purpose which would be compatible with particular chemical fluids, it may not be compatible with others. Moreover, metals chosen for their low thermal coefficient to minimize the effect of temperature changes on the pressure reading may not otherwise have physical characteristics compatible with the fluid being metered.